Conventionally, a wireless communication system operates in a proprietary radio frequency (RF) spectrum in which base stations and wireless terminals communicate through the proprietary RF spectrum licensed to a wireless operator. However, there has been discussions of wireless communication systems expanding usages to unlicensed spectrum, such as the Industrial, Scientific and Medical RF spectrum (ISM band) or other free spectrum. The possibilities of Long Term Evolution (LTE) or LTE-advanced communication systems making incursions into the unlicensed spectrums have drawn attention for telecommunication equipment vendors and operators. One reason for such interest is the potential overcrowding of licensed spectrums. In order to provide high throughput services to more users, incursions into unlicensed spectrums might alleviate overcrowding of for wireless communication systems.
In 2014 September, a new study item, namely “Study on Licensed-Assisted Access using LTE” was approved for investigations related to 3GPP Release 13. The framework for Licensed-Assisted Access (LAA) to unlicensed spectrum has also been known as Unlicensed LTE (LTE-U), which may potentially be a key feature for the next generation cellular system.
One of the challenges is to operate a cellular system in an environment of uncertainty and to consider co-existing issues. As the communications is conducted in unlicensed or shared spectrum, there might be other communications devices that, either using the same radio access technology or different radio access technology, would like to use the same unlicensed spectrum. For example, unlicensed LTE operations might need to co-exist with Wi-Fi radios which operates in an unlicensed spectrum and is not under the direct control of the LTE operations.
Since the unlicensed band is shared by other radio access technology such as Wi-Fi or by another LTE-U transmitter, the planned transmission could be delayed or might not actually be transmitted because of on-going transmissions in the unlicensed band.
One conventional method of transmitting reference signals is that a base station transmits reference signals 101 periodically, as shown in FIG. 1. If the reference signals 101 are transmitted periodically in the licensed spectrum, the reference signals could be received by tuning to a specific frequency spectrum. In an LTE-U operation however, in some occasions reference signals might not be transmitted because the unlicensed spectrum is occupied as one or more other radio transceivers might be using the available unlicensed spectrum at that moment.
Therefore, providing solutions for transmitting reference signals for LTE-U operation would be required.